The present invention relates to a semiconductor laser device fabricating method capable of forming a plurality of semiconductor lasers on one semiconductor substrate.
In recent years, optical disks have been popularized, and their recording formats have had many divergences. When optical disks of different standards are optically read, lasers of different standards are needed. For example, in order to read two types of optical discs of a CD (Compact Disc) and a DVD (Digital Versatile Disc), an infrared laser of an emission wavelength at and around 780 nm and a red laser of an emission wavelength at and around 650 nm are needed.
In the above case, there is demanded the appearance of a semiconductor laser device capable of emitting laser beams of two wavelengths in one package for the size reduction and cost reduction of the pickup.
Moreover, besides the optical disks, there is demanded the appearance of a semiconductor laser device capable of emitting laser beams of two wavelengths in one package or two kinds of lasers for a low output use and a high output use even at same wavelength for laser beam printers and rewritable optical disks. Furthermore, a two-beam laser device of same wavelength and same output can be considered.
In order to meet these demands, a technology for integrating two semiconductor lasers on one semiconductor substrate is developed. However, when lasers of two different characteristics are formed on a single semiconductor substrate, it is often impossible to materialize such a device through one-time crystal growth. Therefore, a method for carrying out crystal growth a plurality of times on a single semiconductor substrate is used. That is, one laser structure is precedingly crystallinically grown on a semiconductor substrate, the other laser structure is formed while being superposedly grown on it, and the succeedingly formed laser structure is removed from the precedingly grown laser structure. When the other laser structure is grown while being superposed on the precedingly crystallinically grown laser structure in the above case, the laser structure crystallinically grown at the first time is partially etched to expose the semiconductor substrate, and the second-time crystal growth is carried out on the substrate.
FIGS. 8A and 8B show the cross section of the device when two semiconductor lasers of an AlGaAs-based semiconductor laser and an AlGaInP-based semiconductor laser are grown on a GaAs substrate 1. First of all, as shown in FIG. 8A, an AlGaAs-based semiconductor laser 9 constructed of an n-type GaAs buffer layer 2, an n-type AlGaAs clad layer 3, an AlGaAs guide layer 4, a multiple quantum well active layer 5, a p-type AlGaAs guide layer 6, a p-type AlGaAs clad layer 7, and a p-type GaAs contact layer (doped with Zn) 8 is grown on an n-type GaAs substrate 1. Then, the AlGaAs-based semiconductor laser 9 is partially removed by etching until the GaAs substrate 1 is exposed.
Thereafter, as shown in FIG. 8B, an AlGaInP-based semiconductor laser 18 constructed of an n-type GaAs buffer layer 11, an n-type AlGaInP clad layer 12, an AlGaInP guide layer 13, a multiple quantum well active layer 14, an AlGaInP guide layer 15, a p-type AlGaInP clad layer 16 and a p-type GaAs contact layer 17 is grown on the entire surface.
As described above, the AlGaAs-based semiconductor laser 9 crystallinically grown at the first time is partially etched to expose the GaAs substrate 1, and thereafter, the second-time crystal growth is carried out. The reason for the above is that two growth interfaces of the first-time growth interface and the second-time growth interface are disadvantageously included in the laser structure formed by the second-time crystal growth unless the removal is effected to the semiconductor substrate.
However, the aforementioned conventional semiconductor laser device fabricating method for carrying out a plurality of times the crystal growth on the single semiconductor substrate has the following problems. That is, as described above, the GaAs substrate 1 is exposed by partially etching the AlGaAs-based semiconductor laser 9 crystallinically grown at the first time. In the above case, removal is effected to the GaAs substrate 1 by concurrently using an etchant of sulfuric acid system or hydrochloric acid system or NH3 system and an etchant of HF system. Then, the second-time crystal growth is carried out on the exposed surface of the GaAs substrate 1.
However, if etching is effected to the semiconductor substrate as described above, there is a problem that the flatness of the etching surface is worsened, and the poor flatness influences the crystal to be grown at the second-time crystal growth, exerting adverse influence on the characteristic surface of the grown semiconductor laser. In particular, when a multiple quantum well layer in which very thin film layers are laminated is used as the active layer of the semiconductor laser, control of film thickness is very important. Therefore, the poor flatness of the groundwork layer exerts enormous influence on the degradation of the characteristics of the multiple quantum well layer.